ANIMAL LIFE AND ANIMAL PRODUCTS. FOODS 435 



out in the stomach, are completed in the small intestine by the 

 trypsin of the pancreatic juice, and the peptones (or amino-acids 

 into which they are split) pass through the intestinal wall into the 

 circulation. The casein of milk, being in colloidal suspension, 

 is completely hydrolyzed to peptones in the stomach. 



When heated, as in cooking, the proteins do not behave alike. 

 Some, like albumen (white of egg) become coagulated, though 

 probably not less digestible. The same is true of the blood pro- 

 teins (haemoglobin, etc.) of beef. On the other hand, the connec- 

 tive tissue of meat (chiefly collagen) is insoluble in cold water, but 

 in hot water goes into colloidal suspension as gelatine. It is 

 therefore softened by judicious roasting (under-done meat), pro- 

 vided the operation is not carried so far (over-done meat) that the 

 water in the meat is largely evaporated. 



Fuel Value. While food is needed primarily to replace the 

 material which is continually eliminated from the system, the 

 organism also requires energy to maintain the routine motions of 

 the heart, intestines, lungs, and other organs, and the normal 

 muscular tension, as well as the movements of the muscles in 

 walking and working. If the heat derived from routine changes 

 is not sufficient to maintain the temperature (37 C.) of the body, 

 then additional food material is oxidized by the system for this 

 specific purpose (compare p. 426). The values of foods are 

 therefore conveniently estimated in terms of the heat they pro- 

 duce when burned their fuel values. 



The average fuel values, as measured in the calorimeter, with 

 certain necessary corrections, and expressed, as is usual in this 

 work, in large calories* per gram, are: Carbohydrates 4 Cal., fats 

 9 Cal., proteins 4 Cal. The fuel values per pound (= 453.6 g.) 

 are 453.6 times greater: Carbohydrates 1800 Cal., fats 4080 Cal., 

 proteins 1800 Cal. 



* One large calorie (1 Cal.) is equal to one thousand small calories (1000 

 cal.), as hitherto defined (p. 162) and used. 



